The present invention relates to a proxy FC (Fibre Channel) port, an FC network system, and an FC transparent transfer method used therefor and, more particularly, to an FC transparent transfer method of transparently transferring FC signals on a WAN (Wide Area Network).
FC is a communication protocol used to build a SAN (Storage Area Network) by connecting a plurality of storage devices and host computers.
The specifications of FC are standardized by “ANSI X3.230-1994, Fibre Channel Physical and Signaling Interface (FC-PH)” and the like.
A wide area SAN can be built by long-distance-connecting two FC devices through a WAN using a technique (FC transparent transfer) of transparently transferring FC signals on an existing WAN. An apparatus which implements FC transparent transfer can be manufactured at a low cost by using simple hardware because there is no need to terminate a protocol higher than the FC-1 layer in converting signals.
A connection example for a network using the above FC transparent transfer will be described with reference to FIG. 12. Referring to FIG. 12, FC devices 41, 44, 45-1 to 45-3, and 46-1 to 46-3 are devices having FC interfaces, e.g., storage devices and host computers.
Each of FC transparent transfer apparatuses 42 and 43 has a function of transparently transferring FC signals on a WAN 400. The FC signal transmitted from the FC device 41 to the FC device 44 is converted into a signal for the WAN 400 by the FC transparent transfer apparatus 42, and transferred to the FC transparent transfer apparatus 43.
The FC transparent transfer apparatus 43 reconstructs an FC signal from the sign al received from the WAN 400, and transfers the signal to the FC device 44. The same processing is performed for the FC signal transmitted from the FC device 44 to the FC device 41.
When a frame is to be transferred between the FC devices 41 and 44, frame loss is prevented by BB flow control (Buffer-to-buffer flow control). BB flow control processing will be described below.
Reception buffers exist in the interface sections of the FC devices 41, 44, 45-1 to 45-3, and 46-1 to 46-3. The maximum number of frames that can be stored in each reception buffer is called BB_Credit (Buffer-to-Buffer Credit).
The two FC devices 41 and 44 connected by FC exchange their BB_Credits by using a predetermined protocol to know the reception buffer sizes of the respective remote devices. Each of the FC devices 41, 44, 45-1 to 45-3, and 46-1 to 46-3 returns a control signal called R_RDY (Receiver Ready) to a transmission source FC device every time one frame is received.
The FC device on the transmitting side counts the number of frames transmitted and the number of R_RDYs received to monitor the remaining amount (remaining capacity) of the reception buffer of the remote device. When the difference between the number of frames transmitted and the number of R_RDYs received reaches the value of BB_Credit in the remote device, there may be no remaining capacity in the reception buffer of the remote device. Therefore the FC device stops transmitting the next frame until R_RDY is received.
Continuous transfer of frames from the FC device 41 to the FC device 44 in the above arrangement of the FC network will be described with reference to FIGS. 13A and 13B.
Assume that both BB_Credits of the FC devices 41 and 44 are “4”, i.e., a maximum of four frames can be stored in each reception buffer. Referring to FIG. 13, the horizontal direction represents the physical position of each device, and the vertical direction represents the time. Each number written on the left side of each drawing indicates the value of the credit counter managed by the FC device 41.
Upon receiving a notification of BB_Credit from the FC device 44, the FC device 41 initializes the credit counter with the value of BB_Credit. The FC device 41 also decrements the credit counter by “1” every time a frame is transmitted, and increments the credit counter by “1” every time R_RDY is received.
When the value of the credit counter becomes “0”, there may be no remaining capacity in the reception buffer of the FC device 44. The FC device 41 stops transmitting a frame.
According to the above conventional FC transparent transfer method, when the FC devices 41 and 44 are short-distance-connected to each other as shown in FIG. 13A, since R_RDYs are returned at a rate sufficiently higher than the frame transmission rate, no problem arises.
If, however, the FC devices 41 and 44 are long-distance-connected to each other as shown in FIG. 13B, the frame transmitted from the FC device 41 arrives at the FC device 44 with a larger delay, and R_RDY returned from the FC device 44 arrives at the FC device 41 with a larger delay.
In the case shown in FIG. 13B, when the fourth frame is transmitted, the credit counter becomes “0”, transmission of the fifth frame is stopped until next R_RDY is received. As described above, as the connection distance between the FC devices 41 and 44 increases, the data transfer rate decrease.
As is obvious from FIG. 13B, if the FC device 44 incorporates a reception buffer large enough for connection distance, no reduction occurs in data transfer rate.
However, the FC interfaces incorporated in storage devices and host computers are designed on the assumption that they are connected to FC switches over short distances, and hence small-capacity reception buffers are used. For this reason, the connection distance is limited to about several km.
Although some interfaces for connecting FC switches to each other have reception buffers large enough for connection over several 10 km, there is hardly any FC interface having a reception buffer large enough for connection over several 100 km or more. In addition, it is impossible to additionally mount a reception buffer in an existing FC device.